Electrical signal processing system



May5,1959. 1 @.EIENZ' 2,885,669

ELECTRICAL SIGNAL PROCESSING SYSTEM Charles E. Lenz,

INE/wwf I-Is Attorney.

C. E. LENZ ELECTRICAL SIGNAL PROCESSING SYSTEM l May 5, 1959 2Sheets-Sheet 2 Filed 001'.. 26, 1953 A a b A IX ,un

22k. bzgl Inventor'. Charles E. Lehz,'

by His Attorhey.

2,885,669 Patented May 5, 195s ELECTRICAL SIGNAL PROCESSING SYSTEM'Charles E. "Lenz, Omaha, Nebr., .assignor to General 'Electric Company,a corporation oli New .York

.Application .October 26, 1953, Serial No. 388,284

1`5'Claims. "(Cl. 343-4171) The present invention relates generally tothe processing of electrical signals and particularly to an arrangementffor processing signals in a predetermined manner with time.

In lthe -elds of seismograp'hy, radar detection, .compu- 'tation,-etc.,a need often arises for processing electrical lsignals as apredetermined vfunction of time. Oftentimes thevfaithfulnessofLprocessing needs to 'be very exact, such thatuncompensated variations vand instabilities in the circuit parameterscannot be tolerated. If a plurality of different signals have to `'be'processed in an identical manner, the problem vvof attaining over alllfaithfulness lof processing vbecomes yeven more dii'cult.

''For-fexample, it is known that in a radar obstacle detection system ofthe t-ype wherein pulses o'ffelectromagfnet'ic lenergy are Vtransmittedl'by adirective lantenna toward objects in space Iand corresponding echopulses from the 'objects are'received fandemployed to determine theposition'of theobject, thepower of the returningecho diminishes'veryrapidly as Ithe object recedes yfrom the radar installation.Quantitatively the pulse echo power returned ;from the target variesinversely as the -fourth power of the range and consequently the-derivedecho voltage varies inversely as the second power ofrange.'To-designarad'ar receiver capable of handling the extremely wide rangeof echo 'voltages without appreciable distortion v*is difficult. Tolcomensate to some extent for lthis rapid `change of echosignal-strength with-range, 'it/is sometimes convenient 'to causeperiodic -variation of the receiver sensitivity in such a manner thatat'the `time-echoes are vreceived 'from 4nearby objects, ther'receivedsensitivityis low, and as 'the :echoes are'receivedtromobjects at increasing ranges, the 'sensitivity increases. Thisaccomplishment results in considerable over-"all simplification ofthereceiver'design. 'The practice of'varying the 1sensitivity of a receiverwith "time -is 'commonly known 'as sensitivity time control, Vortime-varying Kgain control.

One ypreviously developed method employs a xed,ipre 'determined `waveyform for varying the gain of the `intermediate frequency amplifiervstageso'f the radar receiver. It the; gain of each stage could'berelied upon to vary linearly in `relation tothe gain control 'wave form,`the over all instantaneous 'gain would 'vary lin 1an ideal manner.However, the yindividuality of electron :tube character- 'istics'precludes 4such an 'eventuality, and the practically `attainableperformance'talls short of .this ideal. In more velaborate 'obstacledetection arrangements separately available multiple echoes .need to beprocessed, 'that is, 'amplified simultaneously. If an attempt were 'madeto "apply 'the 'above-:mentioned 'method of time-Varying gain to-th'e'separate channelscarryin'g Athese multiple echoes, it

would be *extremely 4ditlicult to Amaintain `the `gain perpetti-allyequal in all `channels. Any variation in the `'parameters in any ofthe=separate channels, due tovo'ltage lconditions, temperature effects,laging etc., would immedi- =ately .'destroy'fthe equalityottime-*varying gain control .in

lthese-channelsand-lead to-distor't-ion of the 'echo information at theoutput of the receiver.

It is therefore an object of my invention to provide an arrangementpermitting identical fprocessingof a'plurality of electrical signals. f

Another object of'myinvention is to accurately process an electricalsignal in a predetermined 'manner-with time.

Another object of my invention is to derive an electrical signal ofpredetermined form.

Another object of ymy invention 'is to `pr"ovide 'anaccurate-time-varyin'g'gain control for multi-'channel' radar systems.

kA further object of my invention is to provide Ia-rnonitoring systemfor accurately controlling the :processing 'of electrical signals in apredetermined manner.

In laccordance with one embodiment of my .invention involvingtime-varying Vgain control of a ydual-'channel radar system, 'the twochannels are -`accommodated by 'applying time-varying gain 'controls vtobut a ksinglefa'mplifier of adequate band Width to Vinclude -the 'echoesI'from rthe multiple channels-at different center frequencies. Theechoes Iare combined to traverse the-common intermediate-frequencyamplifier together 'with a monitoring-frequencylsignal. After separationat-the outputotthe common amplifier, the amplified-monitoringsignal isdem'odulated, and its negative envelope fis compared with a referencesignal having a positive 'time-varying `gain wave form. The resultantcontrol'signal is vfedback with appropriate polarity to the yanip'lifierto ymaintain the instantaneous `gain-of the amplifier lproportionalfatall ltimes Ito the amplitude ofthe reference signal.

Theffeatures offthe present invention which are believed ytof'be novelare setiforth withrparticularityin the appended claims. The presentinvention itself, Iboth as'toits forlganization and manner of operation,together with further objects 'and advantages thereof, may best 'beunderstood by reference to the following description taken in'connection with the accompanying drawings -in which Fig. l is a groupof wave forms `useful in 4explaining lthe operation of a portion of thesystem 'shownin Pigs. "2 and'3;

Fig. 2 is a block-diagramo'f fa radar'object location `sys tem embodyingmy invention; and

Fig. 3 is `a detailed circuit diagramof fcertain elements represented-in block lform in Fig. 2.

In the following description of the radar system 'embodying 'theinvention, certain frequency values lare -assigned at various 'portionsof #the system in order rto facilitate an understanding of itsoperation. It is to tbe understood, however, that such frequencyassignments are employed purely by way yof yexample and are not tto beconstrued in any 'way as limiting fthe scope lof th'e invention.

Reference is now 1made tto Fig. 1d, showing two cycles vof a typicaltime-varying gain Voltage wave form. The repetition period T of thiswave form is the same as 'that -of the transmitted pulse P, Fig. la, "ofIan Object-locating system. The two pulses E, following`each-transmitted pulse, P constitute Vecho pulses received 'asreflections for -reradiations of the transmitted pulses from tworemotely `located objects. The time spacing-ofth'ese echoes from therelated /transmtited pulse corresponds y-to the different ranges of thedetected objects. It l'should be Vnoted that, in accordance with theprevious discussion, the 'amplitudes of the Aechoes E'are smaller'forsuc'cessively lgreater 4Vranges. Inforder that all echoes reflectedfrom equal size `'objects appear at the foutput `of the receiver ywith`equal amplitudes, independently yof the xfrange -of 'the objects, the-gain of the |amplifying stages -in the receiver 'need "to be varied ina predetermined manner with time after each Ypulse transmission P. In anembodiment l'off 'the invention to 'be described shortly, a monitoringsig- 'nal -is rcaused to *traverse lthe controlled #amplifying stagesbeing traversed by the echo signals, and the i stood that the 1 jamplitude of Atheampliiiedl monitoring signal, after ide-1 f modulation,i's'compa-red lvvit'lithej vstandard or referencel 'wave lform 2shovvninFig; ld to derive a resultant jcouf tro'lj signal.'y This latter .signalis rthen lappliedl .to the screen "grids 'of the electroni dischargedevices rcomprising the amplifying stages, rto cause the over-"all gainvoff "the 'f y amplifier to vary' ina .desired parabolic manner withytime.v s The' amount ofl over-all gain variation :is determined vby theamplituder of thefcontrol-voltage wave i f forrn,`while therate'ofchange of gain lduring the ,periody lof increaseis determined byits slope.

In accordance with `normal practice, echo signals .arci f f prevented?from' reaching the amplifying stages iuntil .a t short rtimey interval!2D, after: each fpulse transmission P- l y byl means of a delayed'range-gate' voltage,l `for 'example Aas illustratedA 'in Fig. 1b, whichis normallyl available f j from the 'radar transmitter; This delayedrange gate. l

Ais a devicecommonly' employed inr radar obstacle'r loi f y atiojnsystems :to prevent: the; receiver, from responding' f f toc thetransmitted pulse' fand the. large', Aerho'e's from nearbyobjectslocated closer than a vprescribed minimum range, which would tendto f overload land ythereby' par-v valyze` 'the receiver for' anjappreciable time thereafter.; f

The threat of such voverloailing yand of` consequent? 11;-l cov'ery:difjculty 'makes gating of the echo channels; inv thefreceiver;de:sira,bl e. y 'The' ,range gate' normally hasy f a l `,time durationccorrespending'to the', maximulrnf rang l `the'radar,y so that thereceiver iwill :conveyf only usefuly j echoes? to; the utilizationdevice land become unresponsive i to ynoise signals existing beyond the,maximum range and beforel the next. echo reception gperiod. Itwill beuuderling 'of the gain-co'ntroll voltage iisf not f necessarily lim edto the range-gate.cycledescribedl but,

may fbe. timed 'in whateverv mannerv de 'siretjly to eiect a i'fume-varying' 1, 'gain control 'at a, proper period. The e result ofyvai'yingthe amplifier l'gain .from iaicondilin. of j 'f Iflow'sensitivity fatthe: time of the pulse transmission,Pv

f f to onei of high.r sensitivity: at' 'the prescribed maximum range ofthe, radarf system 'is" lto provide :echoes of jre-fr i .duced'dynamiczamplitude vrange at the output of l'the re- 'ceivenf 'Thus' equalarea-,objects vresult' in equal: ample f tude lechoes Aindependentlylof' range: as shown in Fig. le. If the targets were of unequal areas theechoes available at the output of the receiver would havecorrespondingly unequal amplitudes. The result is improved informationabout the objects being located which is not distorted by echoattenuation with range.

Fig. 2 illustrates an embodiment of the invention in connection with aradar obstacle location system of the monopulse type wherein echoinformation is available in two separate channels. A novel arrangementis provided to vary the amplification of the echo signals available inthe two channels identically and accurately in accordance with aprescribed time-varying gain control voltage of parabolic wave form. Thesystem shown therein for the determination of obstacle ranges comprisesa pulse transmitter 1 of any usual form adapted to radiate directivelyfrom an antenna 2, a train of suitable high-intensity short-periodpulses to obstacles or objects (not shown) at different distances withina predetermined range. In a monopulse system, echoes may be receivedover a plurality of differently directed antennas and processed to yielddesired obstacle-location information. For purposes of discussion weshall assume that two spaced antennas 3 and 4 having diierentdirectivities are employed to recieve the echoes reected or emanatingfrom remotely located obstacles and provide them on two separatechannels for processing into the desirable obstacle-locationinformation. The echoes may be considered as being received by separatereceivers 5 and 6 connected to respective antennas 3 and 4. The echoreceivers 5 and 6 may comprise radio frequency amplifying stages andconverter stages of usual form for yielding respective echo signals Aand B at different intermediate frequencies over channels 7 and 8. In aparticular f embodiment',= l,the intermediate. `frequencies:y oi

echoes A andBwere'centeredlat 14 andlS megacycles; i y i l'per secondrespectively and carried combined amplitude and phase modulation. lBefore.utl..zingthe echo signals available rin the sepa, y ratechannelait is ydesirable ;to realize the Thenetsof' 'time-varying gain controlaspreviolrlslyl explained; :Since lit is ldifficult tomaintain gainsperpetually equal in more echo signals without j sjuthcient.y skirt:terfer'ence, The

than one channel, the two channels are accommodated vin accordance withone embodiment of my; inventiou, by applying time-varying lgain controlto but a vrsingle amplil :tier ofadequate band width to include thermultiple chate; nels at diterent: center frequencies.A The; .channelspass yrst. through 'selective preampliliers, then ycombinel to jtraverse the jcomrnon `intermediate.-frequency amplifier, l f f and arednally separated by selected postamplifiers. Conf sequently, equal gainamplitude canbe maintainedy continuously in: both channels.

f The processing of the electrical signalslthrough a cornmonfarnplierfisachieved in the following manner.-r The 1 1, echo signalsfAand B are applied:r torespective preamplifyv ingy stages 9'and 10 ofiusual formadapted tol amplify the j distortion v while-i-1aving fminimize interehannel ,in-

:arenormally,y inoperative, v and respond only to a rangegfate voltage Ravailable over v i 1, lead 11 from thel pulse transmitter `1for passingthe preamplied echo' signals A andf B toj the adder circuiti 13, l 'f ftof be addedtogether. f The yrange gate voltage 1 R 's' Eof i f 'f jusual lform,'as fl0riexample; a vcon ventional square wave "timed` with'respect; torthe transmission 'of each' pulse by y f transmitter 1. Inapreferred embodiment; the rangeg'ate i voltage `is delayed-,to occur,asmall intervall ofv Atime D 1 35i aftereach` .pulse transmission as;erpllained lpreviously in i i connection, with Fig. ,ibi

prises a gated triangular wave generator in conjunction with either asquarer or an integrator. An improved arrangement employing the lattermethod is described in a copending application Wave Forming Circuits,Charles E. Lenz, Serial No. 376,192 led August Z4, 1953, assigned to thesame assignee.

Generator 15 is synchronized with the operation of transmitter 1 togenerate a reference voltage G starting at the time of each pulsetransmission P and ending at the conclusion of the range gate voltage R.Thus transmitter 1 may comprise a square wave generator triggered at thestart of each pulse transmission for supplying the periodicallyoccurring square waves W shown in Fig. 1c, over lead 12 to generator 15.In response to each of the applied Waves W, generator 15 generates thereference voltage G, of parabolic form as shown in Fig. 1d. Thisreference voltage is then combined with another voltage, to be describedshortly, in adder circuit 16 to yield a desired gain-control voltage.This control voltage is then applied through control amplifier 17 to thecontrol driver stages 18 and 19. Thereupon, the output of the controldriver stages is used to control the gain of the first and secondintermediate-frequency stages of the intermediate-frequency amplifier14. The detailed functioning of circuits 16 through 19 will be describedshortly. Suffice it to say that the echoes C of unequal amplitudeundergo unequal amplifcations in amplifier 14 in accordance with theirdifferent times of arrival at the receivers 5 and 6 as referenced fromthe instant of the related pulse transmissions from transmitter 1. Theparabolic conguration of the reference voltage G is selected s o thatthe amplied echoes C appear at the output of receiver 10 with equalamplitudes on lead 20. The echoes l The composite voltage C availablefrom the adder 1 3 thentraverses a yco'rnlrrnsn intermediate-frequencyampli- 'y v i fier 14ofanydesiredfnurnber of .'stage's.v 'I norder tolobj l f j tain: 'proper-f time-variable: gain. control of the amplifierl l14,' 'a function generator. 15 ofanyvr suitable type is adapted 1 i.to provider a reference voltage G; 'of parabolic fforrn. .A2 l,conventional' device for generating parabolicy waves coml'ly .flowinsuita'bl kforr .for processing ,nto' the desirable lobstclerlocationinformation. The properly` ampli'ed .eclfroesfC'are then `separated in`postarriplifiers'zl and 22 before passingto' the utilization device"23.As, in the icase ofthe preamplifiers 9 and 10, the postarnpliliers aremade properly selective to avoidv nterchannel kinterference andexcessive' distortion. 'The' utilization' device maybe of any form'adapted to extract the desirable -obstacle-location ,informationcontained 'in the, vmodulations of thesignals available in the twochannels.

iItshould' be noted that the probability of achieving identical timeamp'lificationxcontrol of' the echoes availablein separate channels. hasbeen materially improved by resorting to a common intermediaterfrequencychannel ,for processing all of the echoes.' However, it must berecognizedhat the gain'ofeach stage in the common `amplifier still maynot `vary linearly with thevgain-control voltageapplied to that stage.,'The individuality of'electron tubefcharacteristics. and instability ofcircuit parameters, as previously '.mentionecl, preclude such aneventuality.` To insurefthat'the commonintermediatefrequency :tampliiiergain varies1as'a'prescribedfunction of time with Arelative independence'from .electron tube characteristics, and associated circuitirregularities, an arrangementv in accordance with an embodiment y'ofYmy ,invention is provided which lemploys feed-back together vwithcontinuous monitoring of the instantaneous gain.

.The monitoring function is 'achieved 'by' the use of an oscillator 24which supplies-unmodulated amplitude oscillations O over lead 25 tothecommonintermediatefrelquencyarnpliier 14. vThese monitoringvoscillationsare fam'plified in the intermediate-frequency' sta-ge' 14along with echoes C. fThe-monitoring `oscillations are separatedinthe'output stageoffamplifier'14'and'appear as oscillations O"`-ove'rlead 26.1It-should fbe noted 'that the vos'cil- `lationsnow contain anamplitude-modulation envelope dependent upon the time-varying 'gain`control actionof amplifier v14. v'The modulated yoscillations O') arev'ampli- -f'edfirrarnplifier2'7 before being y-demondulated in a con-Aventionalmanner Ain a demodulatorZ to yield a negative- `goingcnvelopewave form Theinstantaneousfamplirude of-"the ldemodulated oscillationsis compared in Athe adder 16 with the *positivel reference voltage G-avail- 'fabler'fmm the -function ygenerator 15. "Theresultant sumvof'vthese- -two signals, indicative ofthe' faithfulness 'ofy theamplification -variationf of amplifier 14 Arelative.y ltoi-the =instantaneous amplitude of the reference-voltage G, is fed rfljac'ltf withappropriate vpolarity-through amplifier -17 land drivers -18 and-"1\9'.to screen grids in' the lgain-control circuits 'of the lamplier14. sutlicientfgain through the =con 'trol'iamplifierf 17 =tends to4make''the-instantaneous gain of amplifier 114' proportional -atvva1l-times to f -the output 'ofi the' function f generator uG.

'Reference isnow Vmade to' the? circuit diagrams of Fig. 3- which'illus'tratefin' greater detail some of -the circuitry :employed-iin theTvarrangement fo'f 'iv-Fig'. 2, Wherever prac- 'Itc'alpthe referencesymbols lemployed in describing Ithe lfunctioning'of'rthe-block-diagram*of-Fig'.` 2 Yare repeated 'in' describingrcorrespondingcomponents of Fig. '3.

The Vadder 13,-'ernp'loyedf-to combine the echoes Aand -'B-'av'ailableifromfpream'plifiers 9 "andi-10 intovthe common-'pulseftrain' C,consists fof-twopentodes' `29 -and- 30 havingtheir. anodes31vfandi'32-:energized through Vresistorchainsf33 and'h 34,respectively#fromfiB-l-.Y The fcathodes' I35 4'and A3,6501"I these tubesarelfconnected through frespectivewresistors '37 and 38 to ground."Pent'odes 29 and "30 ser-ve 1toadd `theAA and B echoes, applied :tovtheirrespective vcontrol grids'39 and 40, )inthe common"plateiloadlcircuit comprising 1center-tapped :primary transformerwinding 41, with sections -polar'izedfashowm vvand #resistor -`42,- andcondenserv 43. The plate load circuit is vdimensioned `to Aprovideasatis'factorily"liatsresponse yover q"theidesired"echo-signal frequencyband when combined "the Lremainingtuned 'circuits l'of the lcommon*Yinter- 4ulxe'cliate'efrequency 'amplifier channel. @Inf a4-preferrednembodiment the signals A andB each occupya Zntiejgcycle-per-esecondrhalf powerband width centeredJat y114.4 and^18"megacycles per second,respectively. "Provision is made in the adder to equalize thegain ofthepentodes A29 andv 30 by adjustment of respective .potentiometers 44 and4.5 connected as voltage dividersbetweenli- 'andground and having'thermovable,taps connected lto they screen grids 46 and .47 of the addertubes.

' The composite signal C developedr in the primary. Atransformer winding41. is coupled through the balanced secondary winding "48, associatedtherewith, latlcLcupling condensers '49 Aand 50'to.. .theinputfelectrodes V5 1 and 75,2 'of the first stage of theintermediate-frequency` arnplier 14. Adjustable condensers 53 andi 54,together with 1 resistor S5 andthe secondarvwihding 48. are tuned toprovide a fiat response over'the desir'ed echo-signalfre- .quency range.whencombined with the remaining'tuned stages of the amplifier k14. Theplate electrodes '5 6, and 57 .of pentodes'.'58 and'59, ,operatingas."diterentiajl`am plifiers, .are energizedithroughthecenter-.tappedtransformer` winding 60, inductance'61,iar'1dresistor,62Vfrom B+. "Thus, amplifiedy versions ofthe push-.pullsignals applied .toIcontrol electrodes,'or grids.` .'51 andlfSZfare .developed across the.tuned 'differential output 'circuit comprising thecenter-tapped.inductance'60, resistor' 63, and thevariable...condensers' 64, and Ibis-output .circuit is tuned tocontribute towards an .overall `fiat vre- .sponse' for'the echoqs-ignalfrequencies.whenconsideed with the .other tuned ,intermediate-.frequency.stages.

"To monitor'the faithfulness Aof'.time:varying gaincontrol exhibited by,.the. intermediate ;fr,eq.uencyfA `arrplijiejr .channel 14, ,amonitoring signal 'oxed amplitude@ I,applied through A a `conunou vinputcircuit-Y .of the.' `differential amplifier tubesgSS 21.11659. 1;Si1.1ceithe..nStan taneous `differential-model gain of `a balanced'dierental amplifier is `at Lalltimes proportional. toi the.instantaneous comon-modegain, the monitoringsignal vin 'being passedthrough the commonmodeacquires an amplitude ,modulation whichjsproportional.A to "..thehinstantaneous"differential andcommon-modeqgains.'ln'.the, presenti-nstance .the monitoringsignal fromoscillator ...24;iS .applied over lead 25 through windinglS .andrcQndenSellS "49 .and '5 0 to the control ,gridsvof .51 and '5,2 tol be.1airiplitied valong with the echoessignals by `tubes ,58 a'r'1d"',59.{In- .ductance 66and condenser '67 QQon'stitutethe commonmode inputcircuitforfthe monitoring 'signal yand Aare accordingly 4tuned ,to lthemonitoring' frequency. The inductance `61, connected between thecenteritapfof .inductance k ,and resistor '62,and condens'erf'68,constitute the common-mode outputcircu'itfor the monitoring ,signal andlconsequently are tun'edto the monitoring frequency. f Condenserj69 acts.as a-gbypass for; alternat ing-.current signals Ato ground. "Thescreenelectrodesv 7,0 -and ,71 vare furnished withgain-control voltageoverlead 72 in a manner to 'be .describedshortly The amplified echosignals available across center- `tapped inductance ,60, nand ytheamplified monitoring'signal available across ,inductance' 61 fareapplied through coupling condensers.'73 Vand v74 to' the control.electrodes 7S and 76 of pentodes '77 and 7S acting .as the seconddifferential `amplifier stage of the intermediate-frequency channel 14.These pentodes lhave ,their plate 'electrodes 79 and 80.energizedthrough center tapped transformer winding 81, finductanceJSZ, andresistor" 83Qfrom'-B-.l--. The screen electrodes B4 andI 85 areenergized withlthe time-varying gain control voltage available `over'`jlefad 86 in a manner to be described shortly. yThe .amplifiedcomposite echo signalsv are developed across the."di f r`ferental outputcircuit comprising .W-indingil, resistor `8,7 and Vcondenser's88Aand89,. These'elements are tunfed to provide the required at frequencyresponse to'the echo signals when considered with the other tuned'intermediate-,frequency circuits.` 'The-amplified echo'sign'als arecoupled throughV the -secondary-transformer'winding 90;associatedwithfthe primarywinding' 81;-to"the"3rd echo signals.

`'tive-going envelope is nWinding 90, resistor 91, and condenser 92 aretuned to contribute to an overallflat frequency response forthe ThisAthird intermediate-frequency stage may be similar to theintermediate-frequency stage comprising pentodes 58 and 59 describedearlier. The output of the third intermediate-frequency amplifier stageis then applied over lead 20, to the postamplifiers 21 and 22, and thento the utilization device as shown in Fig. 2.

The monitoring signal, after amplification by pentodes 79 and 80, isdeveloped across the common-mode output circuit comprising inductance 82and condenser 93 which are tuned to the monitoring-signal frequency.Condenser 94 acts as a bypass for the alternating-current signalsA toground. `rThe amplitude-modulated monitoring sign-al `s effectivelyseparated from the echo signals by the common-mode output circuit andmade available on lead 26 for comparison with the reference voltage G.First, however, amplifier 27 amplifies the modulated monitoring signalO' to a suitable level for demodulation by demodulator 28. The amplifiedmonitoring signal is applied to the control electrode 9S of triode 96,operating as acathode follower. Its anode 97 is connected through aresistor 98 to B+ and its cathode 99 is connected through resistors 100and 101 and condenser 102 to ground. Operating bias for triode 96 isobtained by application of the portion of B- voltage developed acrossresistor 100, between the cathode 99 and the grid 95 through theresistor 103. The monitoring signal available at the cathode 99 is thencoupled through condenser 104 to the rectifying diode 105 acting as thedemodulator. Inductance 106 contributes to a suitable load forcathode-follower triode 96 and is tuned together with its stray capacityto the 4monitoring-signal frequency. Diode 105 rectifies the wave form Oafter amplification in 27 to yield its negative-going envelope aftersmoothing in the low-pass filter circuit comprising inductance 107,condenser 108, and the resistor-condenser combination 109. Aftersmoothingin the low-pass filter, the negaapplied to the Controlelectrode 110 of the triode 111 connected as a cathode follower. Theanode 112 is connected through resistor 113 to B+, while the cathode 114is connected through resistor 115 and condenser 116 to ground. Operatingbias for the triode 111 is obtained from B, connected to the junction ofcondenser 116 and resistor 115.

The negative-going envelope Ol available at the cathode of triode 111 iscoupled through condenser 117 and resistor 118 to the control electrode119 of pentode 120 together with the reference voltage G available fromgenerator through resistor 121. Pentode 120 has its anode 121 connectedthrough the load resistor 122 to B+ and its cathode 123 connectedthrough the parallel resistor-condenser circuit 124 to ground. Pentode120 acts to add the signale G and O" at its anode 121. The resultant sumof these two signals is then coupled through condenser 125 to thecontrol amplifier 17. Resistor 126 couples a portion of the resultanterror or control signal back to the control electrode 119 to improve thelinear response of this circuit.

The control signal, after suitable amplification in amplifier 17, isapplied through condenser 127 and potentiometer 128 to the controlelectrode 129 of triode 130.

Triodc 130 operates as a cathode lfollower with its anode 131 connectedto B+ and its cathode 132 connected by lead 72 to the screen electrodes70 and 71 of the first intermediate-frequency amplifier stage 14. Thus,the control signal resulting from a comparison of the reference voltageand demodulated monitoring voltage is used to control the screenpotential of the first intermediate-frequency amplifier stage, andconsequently the gain thereof. In similar manner, the control signalfrom amplifier 17 is applied through condenser 133 and resistive`voltage divider 134 to the control electrode 135 of triode 136 actingas a cathode follower. The anode 137 of this triode is connecteddirectly to B+ while its cathode 138 is connected'by the lead 86 to thescreen electrodes 84 and 85 ofthe second intermediate-frequencyamplifier stage. Thus the control signal resulting from a comparison ofthe reference voltage and demodulated monitoring voltage is also used tocontrol the voltage applied to the screens of the secondintermediatefrequency amplifier stage, and hence the `gain thereof. p

Control drivers 18 and 19 are provided because the low-frequency controlamplifier 17 is not capable of supplying the moderate screen powerrequired for feed-back control of intermediate-frequency amplifierstages.

In addition, the control drivers 18 and 19 fulfill the more importantfunction of establishing a suitable, selectively fixed,` voltage levelfrom which the lgain control voltage may vary in a positive goingdirection during each of its periods. This voltage is termed thequiescent driver voltage, since it establishes the voltage at the screengrids of the Igain controlled amplifier stages during the time when novariable, gain control voltage is available 'from amplifier 17. Thelevel of this positive, quiescent driver voltage for the grids of tubes130 and 136 is selectively determined by adjustment of the movable tapson potentiometers 141 and 143 connected in respective voltage dividerchains to B+ as previously mentioned. Thus, the positive-going, gaincontrol voltage from amplifier 17 is superimposed on the quiescentvoltage level established by potentiometers 141 and 143 during eachperiod and for the duration of the voltage waveform G available fromgenerator 15. The resultant waveform is transmitted by cathode followeraction of driver tubes 130 and 136 to the screens 70, 71, 84and ofamplifier tubes 58, 59, 77, and 78 to accurately control the overallgain of channel 14 in accordance with the parabolic form off thereference voltage, G, available from generator 15.

Diodes 139 and 140 are connected across potentiometer 128 and voltagedivider 134 to insure that the voltage of the `grids 129 and 135 nevergo below the positive level established by the movable taps onpotentiometers 141 and 143. For example, the diodes are so poled, thatif the voltage at the output sides of condensers 127 and 133 should tendto -go negative with respect to the voltage at the movable taps of 141and 143, they would conduct and clamp the grids 129 and 135 directly tothe quiescent voltage sources 141 and 143.

Potentiometer 128 also acts as a relative-gain control to correlate thedriving voltages supplied to the screens in the two feed-back controlledcommon intermediatefrequency amplifier stages. Although the `gain offthe second control driver 19 is fixed, the gain of the first driver 18is adjustable by means ofthe movable tap on potentiometer 128. Thecontrol drivers just discussed constitute the last necessary link in thedual-channel intermediate-frequency amplifier incorporating time-varyinggain control. When combined as shown in Fig. 3, these circuits form atheoretically satisfactory system whose performance is capable ofproviding `gain-control accuracy of at least 3% in one particularembodiment. This means that under ideal conditions the instantaneousintermediate-frequency gain would deviate from the true parabolic riseby no more'than the same percentage.

Many different settings ofthe relative-gain control 128 can result ingain control demodulator outputs similar to that already mentioned.Nevertheless, this control affects the waveform of the error voltagefrom the control adder and of the screen grid voltages supplied to bothfeed-back controlled common intermediate-frequency amplifier stages.

Although the present invention has been describedin terms of atwo-stage, controlled, or time-varying gain amplifier, in which thecontrol signal, resulting from a comparison of the time-variable gainreference voltage and the demodulated monitoring signal, is applied toboth .'17 ofthe"tun`ction 1generator output `B(t), vthat is, a powerofthe amplitude -of the reference voltage, as lfollows:

a .output signa1=a (one) b where Ila=nurnber of-'feedback-controlledstages traversed by the l inputsignal fb :number of feedback-controlled'stages traversed by the monitoring voltage, and f'(t')indicates -that Aand B may be functions of time, Vand AU) and B'(t) Yare greater thanyzero "Thusin `the arrangement of Fig. 3, A(t) is equal to theappliedecho signals, C, B(t) is theparabolic Wave.- 'form.'G, and

a' .2 Tfr-1 Thus the amplitude of the input signal is made directlyproportional to `the reference voltage. It can be shown that a and b canonly be integers up to and including n, and the ratio m'aylbe equal fto,greater than, or 'smaller than l. If the monitoring and Iinput signalstraverse all ofthe feedback -or gain controlled stages, then thestagesneed not -be identical. However, -if either 'the monitoring -orinput 'signals do not traverse all gain controlled stages, and thestages are different, then/the simple formula whlten-suffice. I-Iowever,with a knowledge of rthe differencesv of these stages, f theoutputsignal may be readily derived. A

'Some of 'the possible, yalthough by no means exhaustive, derivationspossible l.with the fpres'ent invention is Where A(`t)=B(f). The presentarrangement would permit derivation of powe'rsof A0), equal to1o'r1`differing fromfl, dependingon 'the value loffthe ratioV"Or,;'i'f"B'('t.) is some `inverse f* power 'of AU), asv for example 1.,tA.(t)

deviationnfrom this `constant or predetermined output would beindicative Vof a .changeiin thetinput. .or monitorins-signals- Althoughoperation in the manner described tends ".16 only `vto .compensateforcharacteristic .irregularitieslyoi Itheitubes in stages through whichthemonitoring voltage passes, theuse of identical tubes yin allfeedbackcomtrolled stages still tends .to minimize-the significance oftube characteristics vin influencing overalloperation.

Previously it was mentioned that the 'overall gainof theettwocontrolledstages in Fig. 3, canv be caused to be approximately.proportional to the amplitude of .the reference voltage Vfromvgenerator 15. Actually .the overall gain of the 'gain-controlled stagesof at given configuration traversed by the monitoring signal can be madeas nearly proportional to the reference voltage `amplitude as desired,regardless of the circuit characteristics of these stages, by anappropriate-control voltage amplifier gain. It is only necessarytoproperlyproportion the amplitude of the control voltagerelative to theamplitudeof the reference. voltage to achieve a desirable ldegree offaithfulness-of overall rgain control with respect to theV referencevoltage waveform. For example, in the embodiment of Fig.y 3, .it waspossible yto `obtain .a modulated control voltage envelope correspondingwithin at least 3% to .the reference voltage wave shape. By increasingVthe gainof the control amplifier 17. without changing the.function-generator output level, or .more .generally stated, changingthe. gain of the loop com; prising the amplifier 27, demodulator 28,adder 16, amplifier 17 and drivers 18and 19, the accuracy of the overallgain control -of the .intermediate-frequency channel 14 can .belimproved. This is accomplished despite any irregularities orinstabilitiesassociated with the circuit parameters of thelintermediate-frequency channel.

While particular embodiments of the present .inven-V tion have :beenshownandl described,` it will beobvious to those skilled in the art thatchanges and modifications may, be made withoutdepartng .from this.invention in itsbroaderaspects `and therefore the aim -in the appendedclaims is to cover all, such.changes and modifications as fall Withinthe truespirit and 4scope of the invention.

WhatI claim. as new and `desire to secure by Letters Patent of theUnited States is: X

1. .An arrangement for processing a first electrical signal `having agiven characteristic modulation, comprisinga Vsource of a second-signal,an electrical circuit having a:given .number of. cascaded stages, saidcircuit adapted to modulate .said rgiven characteristic ofappliedsignals, means vfor simultaneously applyingsaid first and second.signals tosaid electrical circuit, a source ofa third. electricalsignal having a .similar characteristic modulation, means for removingsaid second signal from one of -said stages, means for comparing thesimilar characteristic .modulationfof said removed second vsignal withsaid similar characteristic modulation of said-.third signal .to derivea control signal, means for applyingsaid control signal toselectedlstages of saidelectrical circuit to modulate vsaid applied firstI andsecond signals Ain accordance with Va similar characteristic of saidcontrol signal, and means for removing said first signal from one vofsaid selected stages.

2. An arrangement for processing a first electrical signal comprising anamplifier having a given number of cascaded stages, a source of a secondelectrical signal having a given time-amplitude characteristic, meansfor simultaneously applying said first and second signals to saidamplier, a sourceof a third electrical signal having a giventime-amplitude'characteristic,fmeans Vfor removing said second signalfromfsaid amplifier after passage through ainumberr of: said stages,ymeans for comparing the amplitudes of, saidy removed second signalandsaid third signal to derive a control signal, means for controlling thegain of selected stages of said amplifier in accordance with theamplitude of ysaid control signal, and means for removing said firstsignal from vsaid amplifier after passage through a number ofsaidselected stages.

3. In combination, means for periodically transmitting pulses ofelectromagnetic energy toward a remote object, a source of electricalmonitoring oscillations of constant amplitude, means for` simultaneouslyreceiving said monitoring oscillations and echo pulses of said trans-Vmeans for varying the sensitivity of said receiving means in accordancewith said control signal.

4. In combination, means for transmitting pulses of electromagneticenergy toward remote refiecting objects, a source of electricalmonitoring oscillations of constant amplitude, means for receiving echopulses of said transmitted electromagnetic energy returned from saidreflecting object and said monitoring oscillations, a circuit forvarying the sensitivity of said receiving means as a predeterminedfunction of time, comprising a source of a signal having aninstantaneous amplitude varying as said function of time and means forcornparing the instantaneous amplitude of said received electricalmonitoring oscillations with the instantaneous amplitude of said signalto derive a control signal, and means for varying the sensitivity ofsaid receiving means in accordance with said control signal.

5. An arrangement for identically processing a plurality of separateelectrical signals comprising a source of a first signal of constantamplitude, a common amplifying channel, means for applying saidelectrical signals and said first signal to said channel for amplifyingsaid first signal and said electrical signals, means for varying thegain of said channel as a given function with time, comprising a sourceof a second signal having an amplitude varying as said function withtime, means p for separating said first signal and electrical signalsafter amplification, means for continuously comparing the amplitude ofsaid separated amplified first signal with said second signal amplitudeto obtain a third signal, and means for modifying the gain of saidchannel in accordance with the amplitude of said third signal.

6. An arrangement for identically processing a plurality of separatelyavailable electrical signals comprising a source of a first signal ofconstant amplitude, a common amplifying channel, means forsimultaneously applying said electrical signals and said first signal tosaid channel for amplifying said first signal and said electricalsignals, means for varying the gain of said channel as a given functionwith time, comprising a source of a second signal having an amplitudevarying as said function of time, means for separating said amplifiedfirst signal from said amplified electrical signals, means for comparingthe amplitude of said separated first signal and said second signal toobtain an error signal, means for modifying the gain of said channel inaccordance with the amplitude of said error signal, means for separatingsaid amplified electrical signals, and means for utilizing saidseparated amplified electrical signals.

7. An arrangement for processing a first signal, comprising anelectrical circuit, a source of a second signal having an unmodulatedamplitude, means for simultaneously applying said first and secondsignals to said circuit to be amplitude modulated, a source of a thirdsignal having an amplitude modulated as a function of time, means foramplitude demodulating said amplified second signal, and means forcomparing the amplitude of said demodulated second signal with that ofsaid third signal to derive a control signal, and means for feeding backsaid control signal with appropriate polarity to said circuit forcontrolling the instantaneous amplitude modulation by said amplifier.

8. An arrangement for deriving an output electrical signal having adesired amplitude modulation comprising a first and a second amplitudemodulated signal, an amplitude modulator circuit, means for`simultaneouslyapply ing said first and second signals to said circuitto be amplitude modulated, a source of a third, amplitude modulatedsignal, means for comparing the instantaneous amplitude of said thirdsignal and said amplitude modulated second signal to derive a fourthsignal, and means for applying said fourth signal to said circuit tomodulate the amplitude of said first and second signals in relation tothe amplitude modulation of said third signal, and means for removingsaid amplitude modulated first signal from said circuit as said outputsignal.

9. An arrangement for identically processing a plurality of separate anddistinct electrical signals comprising a source of a monitoring signalof constant amplitude, a time variable gain control signal, a commonamplifying channel comprising push pull amplifying stages connected incascade means for applying said monitoring signal in push pushrelationship and said electrical signals in push pull relationshipsimultaneously to said stages to be amplified, means for varying thegain of said channel in relation to the instantaneous amplitude of saidtime variable gain signal comprising means for separating said amplifiedmonitoring signal and amplified electrical sig nals, means forcontinuously comparing the amplitude of said separated monitoring signalwith that of said time variable gain signal to obtain an error signal,and means for modifying the gain of said channel in accordance with theamplitude of said error signal.

l0. An arrangement for processing a first electrical amplitude signal Ato derive an output amplitude signal equal to comprising an amplifierhaving a plurality of cascaded stages, a source of a second constantamplitude electrical signal, means for simultaneously applying saidfirst and second signals to said amplifier, a source of a thirdelectrical amplitude signal B, means for removing said second signalfrom said amplifier after passage through a number b of said stages,means for combining the amplitudes of said removed second signal andsaid third signal to derive a differential amplitude control signal,means for controlling the gain of said stages of said amplifier inaccordance with the amplitude of said control signal, and means forremoving said first signal from said amplifier after passage through anumber a of said gain controlled stages.

ll. In combination, means for transmitting pulses toward a remoteobject, a source of monitoring Waves of constant amplitude, means forreceiving said monitoring waves and the transmitted pulses returned fromsaid object, a circuit for periodically varying the sensitivity of saidreceiving means as a predetermined function of time, said circuitcomprising a source of signals having an instantaneous amplitude duringeach period varying as said function of time, and means for comparingthe instantaneous amplitude of said received monitoring Waves with theamplitude of said signals to derive a control signal, and means forvarying the sensitivity of said receiving means in accordance with saidcontrol signal.

12. An arrangement for processing a first electrical signal An to derivean output signal equal to where n is any positive or negative integer,said arrangement comprising an amplifier having a plurality of cascadedstages, a source of a second electrical signal having a constantamplitude, means for simultaneously applying said first and secondsignals to said amplifier, a

source of a third electrical signal B11, means for removing ,'saidsecond signal from said amplifier after passage through a number b ofgain controlled amplifier stages, means for comparing the amplitudes ofsaid removed second signal and said third signal to derive a controlsignal, means for controlling the gain of selected stages of saidamplifier in accordance with the amplitude of said control signal, andmeans for removing said first signal from said amplifier after passagethrough a number a of said gain controlled amplifier stages.

13. An arrangement for processing a rst electrical signal A having agiven recurrence rate to derive an output signal equal to l AB saidarrangement comprising an amplifier having a plurality of cascadedstages, a source of a second electrical signal having a constantamplitude, means for simultaneously applying said first and secondsignals to said amplifier, a source of a third electrical signal B, ofsaid given recurrence rate, means for removing said second signal fromsaid amplifier after passage through a number b of gain controlledamplifier stages, means for comparing the amplitudes of said removedsecond signal and said third signal to derive a differential amplitudesignal, means for controlling the gain of selectedstages of saidamplifier in accordance with the amplitude of said differential signal,and means for removing said first signal from said amplifier afterpassage through a number a of said gain controlled amplifier stages.

14. In combination, means for transmitting waves toward a remote object,a source of monitoring waves having a given time-amplitudecharacteristic, means for receiving said monitoring waves and thetransmitted waves returned from said object, a circuit for recurrentlyvarying the sensitivity of said receiving means as a predeterminedfunction of time, said circuit comprising a source of signals having aninstantaneous amplitude during each recurrence rate period which variesas said function of time, and means for comparing the instantaneousamplitude of said received monitoring waves with the amplitude of saidsignals to derive a control signal, and means for varying thesensitivity of said receiving means in accordance with said controlsignal.

15. In combination, means for transmitting waves toward a remote object,a source of monitoring waves having a given time-amplitudecharacteristic, means for receiving said monitoring Waves and thetransmitted waves returned from said object, a circuit for varying thesensitivity of said receiving means, said circuit comprising a source ofamplitude modulated signals and means for comparing the instantaneousamplitude of said received monitoring waves with the amplitude of saidsignals to derive a control signal, means for varying the sensitivity ofsaid receiving means in accordance with said control signal.

References Cited in the file of this patent UNITED STATES PATENTS2,422,069 Bedford June 10, 1947 2,422,334 Bedford June 17, 19472,564,694 Huber Aug. 21. 1951

